The goal of this research is to contribute to our understanding of the neuronal machinery that is responsible for vasopressin release by the hypothalamic magnocellular neuroendocrine cells. We propose to study the response characteristics of the whole neuronal population by measuring plasma arginine vasopressin in the blood of cat, rat and monkey and of indvidual neurons by recording activity of antidromically identified single neurons in cat, rat and monkey. We suggest that these studies will further define the electorophysiological characteristics of hypothalamic vasopressinergic neuroendocrine cells and their role in the homeostasis of body fluids.